Adults = 1 unit, girls = 3 units. Boys are 120 fewer than girls, so boys = 3 units − 120. The three groups total 846.

Adults1Girls111Boys111girls − boys = 120

Plain numbers are given; italic underlined numbers are worked out.

  1. Adults + Girls + Boys = 1 + 3 + 3 units − 120 = 7 units − 120 → 846
  2. 7 units → 846 + 120 = 966
  3. 1 unit → 966 ÷ 7 = 138 (adults)
  4. Boys → 3 × 138 − 120 = 414 − 120 = 294
  5. Difference boys − adults → 294 − 138 = 156

The difference between boys and adults is 156.

Check: adults 138 + girls 414 + boys 294 = 846. ✓